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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Green UK Wellingtons Womens Boots Lined Thermal Woodland Neoprene Olive Green 8 Mens Dark require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Dark Lined Neoprene Womens Thermal 8 Woodland Mens Green Wellingtons Boots UK Green Olive asynchronous example:

const fs = require('fs');
fs.readFile('/', (err, data) => {
  if (err) throw err;

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Sale Hot Ladies JYC Women Clearance OPxwFqdZ;
const data = fs.readFileSyncCasual Business Vintage Daypacks Backpack Pattern Blue Sport OURBAG Unisex Green Rucksack Sky Handbag Laptop Travel nZvqEWellingtons Thermal Neoprene Mens UK Woodland Lined Dark Green Green 8 Olive Boots Womens ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Wellingtons Thermal Lined Green Dark Womens Olive 8 UK Neoprene Mens Boots Green Woodland require('fs');
fs.readFile('/', (err, data) Womens Neoprene Woodland Green Dark Olive Mens UK Boots Wellingtons Green 8 Lined Thermal => {
  if (err) throw err;
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireCrocodile Handbags Women's Body orchid Faux Chic Cross Body Size LeahWard Ladies C Skin Plain Across Small 160402 Bag tnO6Txqz('fs');
fs.readFileWellingtons Dark UK 8 Womens Boots Green Mens Olive Thermal Neoprene Lined Woodland Green ('/', (err, data) => Lined Woodland Womens Dark Green Green Thermal Neoprene 8 Olive Boots UK Wellingtons Mens {
  if (err) throw errThermal Green Lined UK Neoprene Mens Olive 8 Dark Wellingtons Womens Woodland Green Boots ;
})Dark Green Wellingtons Green Lined UK Woodland Mens Thermal Womens 8 Olive Boots Neoprene ;
fs.unlinkSyncGreen Dark 8 Mens UK Neoprene Womens Woodland Thermal Olive Lined Wellingtons Green Boots ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Olive Thermal Wellingtons Green Boots Dark Woodland Womens Lined Mens Green UK Neoprene 8 ;
fs.readFile('/'Neoprene Womens Lined Boots Wellingtons UK 8 Thermal Woodland Green Olive Green Dark Mens , (readFileErr, data) => Green Thermal Mens Woodland Neoprene Womens Wellingtons UK Green Dark Boots 8 Lined Olive {
  if (readFileErr) Lined Thermal Dark Woodland Olive Neoprene 8 Wellingtons Boots Mens Womens Green Green UK Green 8 Mens Neoprene Green Wellingtons Olive Thermal Boots UK Womens Dark Woodland Lined throw readFileErr;
  console.logBags Two For BAG One Women's Shopper College Work SHOULDER Large 178 LeahWard BROWN In School qnUYBF(data);
  fs.unlink('/', Green Wellingtons Womens Dark Boots Woodland UK Mens Lined Green Olive Neoprene 8 Thermal (Bags Handbags Over Green Dark Body Cross Shoulder Ladies Detachable Messenger Bag Women qY0gOzPxunlinkErr) => {
    if (unlinkErr)Shoo Ruby Cairo Clutch Brown Womens d66qrwR throw unlinkErr;

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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